Drill stem safety valves (“DSSV”) typically have two primary purposes: a) they are safety devices that can be closed to prevent mud and/or well fluid from flowing back up the interior of the drill pipe in the event of an unbalanced pressure in the mud column; and b) they can be used as a flow control device to turn on and off the flow of mud while making and breaking connections during drilling operations for top drives.
When used for blow out prevention, these valves are only used during testing or in emergencies. However, in mud control, they can be operated several hundred times in the drilling of a single well.
Drilling mud is an abrasive, highly engineered fluid that is used to balance pressure in the string against pressure in the pay zone upon point of penetration. The abrasiveness of the fluid is due to entrained solids such as sand. Well fluids are any hydrocarbons in the pay zone, and can include a mixture of oil, gas and solids.
To operate a DSSV, it is simply turned from the open to closed position and back again, by applying torque to the DSSV stem. This torque can be applied manually, or by an actuator. The stem in the valve is a part which penetrates the pressure envelope. It typically has a hexagonal interface to receive a wrench that can be used to open or close the valve.
The body of the valve is the part which houses all the internal parts. The body is typically constructed of high strength carbon steel alloy due to the extremely high pressure, torque and tension it is subjected to as part of the drill string. This type of alloy is selected for its strength, but as such is not very corrosion resistant and cannot be welded because welding introduces localized hardening that can cause premature failure under high loads.
Under pressure, the valve takes a significant amount of torque to operate. Some valves require upwards of 2000 foot-pounds (“ft-lbs”) to operate. This torque can be applied manually by wrench, or by actuator. Because of this, there is substantial load on the hexagonal stem, and conversely when the valve reaches its full travel, this load is transferred to the “valve stop”.
The stops in the valve are the contact areas between the stem and, typically, a stop ring or the body itself. The stops must have enough surface area to withstand the load applied either by wrench or by actuator. The ability of these stops to handle the loads applied is critical to proper function of the valves. If the stops are too weak, i.e. there is not enough “stopping power”, they will quickly yield under load. As the stops yield, they allow the ball to travel further and further from its optimal position in both the open and closed direction. This is referred to as “over travel”. Any over travel can have significant, negative effects on valve performance and life, and can result in infantile failure.
Currently, for any DSSV, the correct alignment of the ball in the open and closed position is critical to optimal valve life. Without correct alignment in the open position, the leading edge of the ball and the trailing edge of the lower seat will be exposed to abrasive mud flow, causing premature wear and potentially vortices that can accelerate erosion. The resulting deflected flow path and resulting accelerated erosion can lead to infantile failure.
There are several methods to ensure alignment of the ball in the open and closed positions. Early stop systems incorporated a “stop ring”. This ring is a removable ring on the interior of the valve exposed to the drilling fluid, usually adjacent to the upper seat, which provides a flat surface for the stem to come into contact with. Due to the nature and design of the ring, the amount of surface area available to stop against is typically very low. Because of the low amount of surface area, these stop rings cannot resist significant amounts of torque and therefore do not have much stopping power. After only a few uses, any yielding in the stop area will allow the ball to over travel in the open and closed direction. However, one advantage of using stop rings is that they are replaceable. Any yielding of the ring can easily be fixed by simply replacing the ring during regular service.
To improve stopping power, “cam style” stops were invented. These types of stops are typically used in higher pressure valves, and are typically single sided (as opposed to dual stops mentioned below). Typically, these types of stops comprise a cam lobe incorporated into the stem, which mates with a cam feature milled directly into the body. The utilization of a cam allows for much more surface area in the stop, resulting in more accurate, reliable and repeatable alignment under high torque applications. However, by incorporating or integrating the stop into the body itself, either by machining or fabricating the stop in the valve body, they are difficult to inspect, not very corrosion-resistant and non-repairable because they cannot be welded. If the body wears out, it must be replaced which shortens the useful life of the valve at great expense.
A recent innovation to cam style stops is the Dual Stop™ stem, as manufactured by Hi-Kalibre Equipment Limited of Edmonton, Alberta, Canada. In this design, the stem lobes are doubled, for double the stopping power. While this provides even further reliability and improved life in the field, these lobes are still difficult to inspect, not very corrosion resistant and when they eventually wear out, they cannot be repaired.
It is, therefore, desirable to provide a stop mechanism for DSSVs that overcomes the shortcomings of the prior art.